i) Field of the Invention
The present invention relates to a memory device with a redundant disk array connected to a computer and a disk cache for storing data read out of disk units of the disk array.
ii) Description of the Related Arts
An array disk device has been known to the public since the University of California at Berkeley (UCB) in the U.S.A. published a thesis about this, and recently a number of documents and patents concerning this array disk system have been published. FIG. 1 shows a conventional memory device explained as raid level 5 in "A Case for Redundant Arrays of Inexpensive Disks (Raid)", by David A. Patterson, Barth Gibson and Randy H. Kats, UCB Report No. UCB/CSD 87/391, December, 1987. In FIG. 1, a disk array device 1 includes four disk units 2a, 2b, 2c and 2d and an exclusive OR arithmetic unit 3 for producing new CK (check) data. The disk units 2a, 2b, 2c and 2d store respective data 4a, 4b and 4c and CK data 5 and constitute a redundant group. The CK data 5 represents an exclusive OR calculation result of the data 4a, 4b and 4c. In this case, for example, any data 4a in the redundant group can be produced by an exclusive OR of the other data 4b and 4c in the same redundant group and the CK data 5. Hence, when the data within any one disk in the same redundant group becomes corrupted, the corrupted data can be reproduced from the other data within the other disks and the CK data to realize a recovery function.
Next, the operation of the above-described conventional memory device will now be described in connection with FIGS. 2 and 3. FIG. 2 is a schematic conceptional view of the processing of the memory device shown in FIG. 1, and FIG. 3 shows a flow chart of the processing of the same.
First, a data readout from the disk array device 1 is carried out in the same manner as a usual disk device. In turn, a data write processing operation is performed as follows. That is, first, new data 6 are received from a computer 10 in step S1, and old data 7 are read out of the disk unit 2a in step S2. Next, old CK data 8 are read out of the disk unit 2d in step S3. After the necessary data are prepared as described above, the exclusive OR arithmetic unit 3 executes an exclusive OR calculation by using the new data 6, the old data 7 and the old CK data 8 to produce new CK data 9 in step S4. Then, the new data 6 are written into the disk unit 2a in step S5 and the new CK data 9 are written into the disk unit 2d in step S6. As a result, the processing is finished.
As described above, in the conventional memory device, in order to produce the new CK data 9 when writing data into the disk array device 1, the old data 7 and the old CK data 8 are read out of the respective disk units 2a and 2d and the readout data are fed into the exclusive OR arith-metic unit 3 to carry out the exclusive OR by using the new data 6, the old data 7 and the old CK data 8. Then, the obtained new CK data 9 and the new data 6 are written into the respective disk units 2d and 2a. Hence, in this case, at the data writing time, four accesses into the disk units are required and thus the data writing speed is slow , which is an obstacle to high speed operation.